The present invention relates to a contact
photocathode of photoelectric tubes produced by a metallic deposit on a substrate intended to receive the said photocathode. It also relates to a manufacturing method for such a contact device.
The invention has a particularly advantageous application in the field of fast image intensifying tubes and ultra-fast slot scanning camera tubes.
The technical problem to be solved for any photocathode contact device for photoelectric tubes, and particularly for fast and ultra-fast tubes, consists in obtaining a fast supply of electrons to the photocathode from the power supply of the tube in order to reduce the time constant RC associated with the photocathode, R being the resistance per unit area of the said photocathode. During operation, the photocathode in fact quickly exhausts itself of photo-electrons, such that it is absolutely essential, if it is desired to operate at high switching frequencies, to regenerate the electrons of the photocathode in a very short time. A known solution to this technical problem is to form a metallic deposit on a substrate intended to receive the said photocathode, for example a semi-transparent conductive sub-layer made from nickel, nickel-chrome, aluminium or palladium. The quantum efficiency of this known contact device is then limited, on the one hand, by the sensitivity of the photocathode itself and, on the other hand, by the optical transmission of the conductive sub-layer/substrate assembly. The thickness of this sub-layer is therefore chosen with the intention of providing a correct compromise between the resistivity of the assembly of the two layers (metal and photocathode) and the optical transmission of the sub-layer and of the photocathode. In other words, the thickness of the conductive sub-layer must be sufficient for its resistance per unit area to be rather low, (typically 100 to 500 .ANG.), without however being so great that the quantum efficiency of the device would be unacceptably reduced. In practice, the necessity for this compromise prevents the obtaining of a contact device which is entirely satisfactory, particularly for ultra-fast tubes. Furthermore, this known type of contact device has the disadvantage, because of the relatively high resistivity of the conductive sub-layer, of efficiently injecting electrons only from the periphery of the photocathode.